1. Field of the Invention
The invention relates generally to the field of disc drives, and more particularly to an apparatus and method for providing a reliable characterization of hydrodynamic grooves in a disc drive.
2. Description of the Related Art
Disc drives are capable of storing large amounts of digital data in a relatively small area. Disc drives store information on one or more recording media. The recording media conventionally takes the form of a circular storage disc, e.g., media, having a plurality of concentric circular recording tracks. A typical disc drive has one or more discs for storing information. This information is written to and read from the discs using read/write heads mounted on actuator arms that are moved from track to track across surfaces of the discs by an actuator mechanism.
Generally, the discs are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the discs under the read/write heads. The spindle motor generally includes a shaft fixed to a base plate and a hub, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the hub interact with a stator winding on the base plate to rotate the hub relative to the shaft. In order to facilitate rotation, one or more bearings are usually disposed between the hub and the shaft.
Over the years, storage density has tended to increase and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage discs. For example, to achieve increased storage densities the read/write heads must be placed increasingly close to the surface of the storage disc. This proximity requires that the disc rotate substantially in a single plane. A slight wobble or run-out in disc rotation can cause the surface of the disc to contact the read/write heads. This is known as a xe2x80x9ccrashxe2x80x9d and can damage the read/write heads and surface of the storage disc resulting in loss of data.
From the foregoing discussion, it can be seen that the bearing assembly which supports the storage disc is of critical importance. One typical bearing assembly comprises ball bearings supported between a pair of races which allow a hub of a storage disc to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor because of low damping.
One alternative bearing design is a hydrodynamic bearing. In a hydrodynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disc hub. In addition to air, typical lubricants include oil or ferromagnetic fluids. Hydrodynamic bearings spread the bearing interface over a large surface area in comparison with a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeat run out.
Another alternative design which has been used with success is a hydrodynamic groove disposed on journals, thrust, and conical hydrodynamic bearings. The hydrodynamic grooves provide a transport mechanism for fluid or air to more evenly distribute fluid pressure within the bearing, and between the rotating surfaces. The shape of the hydrodynamic grooves is dependant on the pressure uniformity desired. For example, a sinusoidal hydrodynamic groove provides a different pressure distribution than a herringbone or helix shaped hydrodynamic groove patterns. The quality of the fluid displacement and therefore the pressure uniformity is generally dependant upon the groove depth and dimensional uniformity. For example, a hydrodynamic groove having a non-uniform depth may lead to pressure differentials and subsequent premature hydrodynamic bearing or journal failure. Generally, to maintain an acceptable hydrodynamic groove shape, a plurality of metrology equipment such as surface analyzers and optical CMM""s are used to analyze the hydrodynamic grooves. Typically, to finalize the hydrodynamic groove measurement the bearing and/or journal are often cut apart to expose a cross-sectional view of the hydrodynamic groove for analysis. Unfortunately, the analysis process is very time consuming and subject to subjective decisions of the hydrodynamic groove dimensional errors which may lead to higher disc drive manufacturing costs. Further, the cutting process may damage the hydrodynamic grooves exacerbating analysis errors. Accordingly, due to the analysis uncertainty, the hydrodynamic grooves may be incorrectly characterized which may lead to premature failure of the bearing and/or journal.
Therefore, a need exists for a method and apparatus to provide a reliable and non-invasive method for characterizing hydrodynamic grooves that is efficient and cost effective.
The invention generally provides a method, and apparatus for characterizing the surface topology of a hydrodynamic bearing. In one embodiment, the invention provides a method of characterizing the topology of one or more hydrodynamic bearing surfaces by rotating the one or more hydrodynamic bearings, then measuring the surfaces of the one or more hydrodynamic bearings. The surface topology is further characterized by determining at least one reference surface and then establishing dimensions of at least one feature disposed on the one or more hydrodynamic bearing surfaces.
In another embodiment, the invention provides a method of analyzing the surface of a hydrodynamic bearing of a disc drive having hydrodynamic grooves by rotating the hydrodynamic bearing along a longitudinal axis and then during rotation, measuring the surface topology of the hydrodynamic bearing. Upon measuring the surface topology, the method determines the angular position of the hydrodynamic grooves along the circumference of the hydrodynamic bearing.
In another embodiment, the invention provides a method for measuring the topology of at least one outer surface of a hydrodynamic bearing disposed about a shaft on a disc drive, comprising a means for measuring the surface topology, and a means for determining the dimensions of features disposed upon the hydrodynamic bearing.